Penalties for growing and selling cannabis must be toughened because a surge in the trade is driving up shootings and gang-related violence, a senior police officer has warned.

Assistant Chief Constable Andy Ward of Merseyside police, who chairs the North West Regional Organised Crime Unit, says the punishments for those caught cultivating and dealing in cannabis are not a deterrent and the drug is now causing greater problems for police than class As.

According to Ward, an "explosion" in cannabis production has resulted in bitter struggles between rival gangs keen to exploit the ease by which cannabis can be manufactured and what they regard as easy money. Shootings in Merseyside have soared by a third since April and officers believe their experience is mirrored across the regions.

Ward, who also heads the force's elite Matrix unit that works to combat drug and gun crime, said: "A lot of these shootings are linked to activity around cannabis.

"We are seeing big increases in cannabis production on Merseyside. Individual groups are fighting turf wars … We have huge issues around cannabis."

Over the past three years, more than 5,120 cannabis factories containing in excess of 345,000 plants with an estimated annual yield of £560m have been discovered across north-west England.

In March, a month-long operation involving officers from six forces targeting cannabis farming resulted in the seizure of cannabis with a street value of nearly £9m. In Merseyside alone, police arrested 147 people over the four-week period.

Ward said: "There has been an explosion in the market for cannabis. Unlike class A drugs, which the criminals can't make themselves, there is the opportunity to grow cannabis in the bathrooms or bedrooms of houses. They can make a lot of money very quickly at less risk [to themselves] and less risk in terms of sentencing.

"Criminals who have previously been involved in something else are drifting into the cannabis world … The amount of money being made by criminals should be reflected in the sentencing."

While the maximum terms for cultivating and supplying cannabis are not dissimilar to equivalent offences involving class A drugs, in practice the sentencing lengths differ vastly. For instance, in May last year, Stuart Thompson was jailed for 15 months after allowing a £2m-a-year cannabis factory to be set up in a barn he owned near Widnes. Alan and Ian Farley, two Liverpool brothers who were part of a £2m heroin operation, were each sentenced to 14 years in January 2011.

Five years after 11-year-old Rhys Jones was shot on his way home from football practice, shootings and gang-related activity account for a small proportion of crime on Merseyside. But, as Ward himself admits, the impact on people's lives and local communities is huge.

Residents in Norris Green, Liverpool, have first-hand experience of the devastating consequences of gun crime. In the early hours of 13 June, Joseph Thompson, 32, was shot and killed opposite Scargreen sports ground.

One woman who has lived in the area for 32 years described how Thompson died outside her front gate. "It was late and I thought I heard someone knocking on the door so I went to see.

"I opened the door and a man was falling to his knees. Someone said I should call an ambulance so I did. I heard someone shout 'He's dead, he's dead'.

"The ambulance hadn't turned up so I called again. The police turned up and said the ambulance was on its way. The man was just lying there. After a couple of hours they put a blanket over him. By 5am they had erected a tent."

The woman, who wished to remain anonymous, said it was frightening: "I couldn't leave my house for two days because the forensics were doing their work. I feel like I'm walking over his grave every time I go out my front door."

Three men have been arrested and bailed over the shooting of Thompson. But his death has had seemingly little impact on gun violence.

Less than two months after Thompson was killed, four teenagers in Bootle received treatment for shotgun injuries. The violence, believed to be part of a feud between two rival gangs, was the fifth shooting in the area in just over a week.

Last week, four people were arrested after shots were fired in the back yard of a Liverpool house. After the shooting in Tuebrook, central Liverpool, a man was admitted to hospital with a gunshot wound to his arm. At the weekend, another man was shot in the back in Toxteth.

In total, there have been 68 firearms discharges in Merseyside since the beginning of April. But the statistics do not tell the whole story. While some troubled districts embody the 1980s stereotype of a broken society – rusted supermarket trolleys, boarded-up terraces, abundant litter – community leaders believe there are reasons to be optimistic.

Lee Donafee is the network director for the Merseyside Inclusion Network and runs the Croxteth Gems, a centre that helps young people realise they have an alternative to a life of crime.

"I know for a fact we are making headway because of how popular the centre is," Donafee said. "We guide and support the kids. It's been hard, I was funding it myself at first, but last year it really took off. The kids say it's easy money to go round selling drugs. We make them aware of what can happen if they follow the wrong path."

Michael Waldock, 16, has been coming to the centre for the past two years. "Coming here brings new opportunities, we go out to places and we can get jobs and work through the centre," he said. "If I wasn't here, I'd be sitting on the street, walking around not doing much at all."

Despite the rise in gang-related violence, Merseyside police says things can improve. It cites Terriers, a new play designed to illustrate the stark reality of becoming involved with gangs and guns, which has been seen by more than 40,000 children.